Unveiling the Affect of Annealing Temperature on the Phase Transformations of Inconel ۷۱۸ Superalloy Manufactured by Additive Manufacturing

Phase transformations and the evolution of hardness during elevated-temperature annealing of Inconel ۷۱۸ superalloy manufactured by the laser powder bed fusion (L-PBF) were investigated. The microstructural evolution, elemental analysis, phase formation, and hardening were characterized by scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and Vickers indentation test, respectively. It was observed that the effect of annealing treatments is directly governed by the annealing parameters (i.e. time and temperature), for which the hardness measurement as a fruitful and convenient tool can reveal this effect. The increase of the hardness, which was obtained by the annealing (aging) treatments at the temperature range of ۸۰۰-۹۰۰ °C, indicated precipitation of the Ni۳Nb γ˝ strengthening phase; while owing to the coarsening of precipitates as a results of overaging at this temperature range, the hardness decreased. For instance the length and aspect ratio of precipitates in the aged sample at ۸۰۰ °C for ۱ h is ۶۷.۱۴ nm and ۰.۳۲, respectively; while these values in the aged sample at ۸۰۰ °C for ۸ h is ۷۸.۳۴ nm and ۰.۴۴, respectively. On the other hand, the decrease of the hardness at temperatures of ۹۵۰ and ۱۰۰۰ °C was attributed to the decrease of dislocation density in conjunction with the Ni۲Nb Laves phase dissolution. Hence, it is crucial to determine the annealing parameters according to the required microstructure and properties.

Keywords:

Nickel-based superalloys , Additive manufacturing , Aging heat treatment , Homogenization treatment , Precipitation strengthening